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Chapter 3 Graphic Methods for Describing Data

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2 Basic Terms A frequency distribution for categorical data is a table that displays the possible categories along with the associated frequencies or relative frequencies. The frequency for a particular category is the number of times the category appears in the data set.

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3 Basic Terms The relative frequency for a particular category is the fraction or proportion of the time that the category appears in the data set. It is calculated as When the table includes relative frequencies, it is sometimes referred to as a relative frequency distribution.

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4 Classroom Data This slide along with the next contains a data set obtained from a large section of students taking Data Analysis in the Winter of 1993 and will be utilized throughout this slide show in the examples.

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5 Classroom Data continued

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6 Frequency Distribution Example The data in the column labeled vision is the answer to the question, “What is your principle means of correcting your vision?” The results are tabulated below

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7 Bar Chart – Procedure 1. Draw a horizontal line, and write the category names or labels below the line at regularly spaced intervals. 2. Draw a vertical line, and label the scale using either frequency (or relative frequency). 3. Place a rectangular bar above each category label. The height is the frequency (or relative frequency) and all bars have the same width.

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8 Bar Chart – Example (frequency)

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9 Bar Chart – (Relative Frequency)

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10 Another Example

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11 Comparative Bar Charts - Example Consider the vision correction for each of the two genders. We then have the following contingency table.

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12 Comparative Bar Charts – Example To compare the vision correction for each gender, we should use relative frequencies because the groups are not the same size. So the following table of relative frequencies is used to draw the comparative bar chart.

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13 Comparative Bar Charts – Example

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14 Pie Charts - Procedure Draw a circle to represent the entire data set. For each category, calculate the “slice” size. Slice size = category relative frequency x 360 Draw a slice of appropriate size for each category.

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15 Pie Chart - Example Using the vision correction data we have:

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16 Pie Chart – Another Example Using the grade data we have:

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17 Dotplots - Procedure 1. Draw a horizontal line and mark it with an appropriate measurement scale. 2. Locate each value in the data set along the measurement sfcale, and represent5 it by a dot. If there are towo or more observations with the same value, stack the dots vertically.

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18 Dotplots - Example Using the weights of the 79 students To compare the weights of the males and females we put the dotplots on top of each other, using the same scales.

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19 Stem and Leaf A quick technique for picturing the distributional pattern associated with numerical data is to create a picture called a stem-and-leaf diagram (Commonly called a stem plot). 1.We want to break up the data into a reasonable number of groups. 2.Looking at the range of the data, we choose the stems (one or more of the leading digits) to get the desired number of groups. 3.The next digits (or digit) after the stem become(s) the leaf. 4.Typically, we truncate (leave off) the remaining digits.

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20 Stem and Leaf Choosing the 1 st two digits as the stem and the 3 rd digit as the leaf we have the following For our first example, we use the weights of the 25 female students.

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21 Stem and Leaf Typically we sort the order the stems in increasing order. We also note on the diagram the units for stems and leaves Stem: Tens and hundreds digits Leaf: Ones digit Probable outliers

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22 Stem-and-leaf – GPA example The following are the GPAs for the 20 advisees of a faculty member. GPA If the ones digit is used as the stem, you only get three groups. You can expand this a little by breaking up the stems by using each stem twice letting the 2 nd digits 0-4 go with the first and the 2 nd digits 5-9 with the second. The next slide gives two versions of the stem-and-leaf diagram.

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23 Stem-and-leaf – GPA example Stem: Ones digit Leaf: Tenths and hundredths digits 1L 1H 2L 2H 3L 3H 65,75 04,22,26,27 66,69,74,80 09,13,15,23,38 50,70,72,89,94 1L 1H 2L 2H 3L 3H Stem: Ones digit Leaf: Tenths digits Note: The characters in a stem-and-leaf diagram must all have the same width, so if typing use courier.

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24 Comparative Stem and Leaf Diagram Student Weight (Comparing two groups) When it is desirable to compare two groups, back-to-back stem and leaf diagrams are useful. Here is the result from the student weights. From this comparative stem and leaf diagram, it is clear that the males weigh more (as a group not necessarily as individuals) than the females

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25 Comparative Stem and Leaf Diagram Student Age From this comparative stem and leaf diagram, it is clear that the male ages are all more closely grouped then the females. Also the females had a number of outliers. female male

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26 Frequency Distributions & Histograms When working with discrete data, the frequency tables are similar to those produced for qualitative data. For example, a survey of local law firms in a medium sized town gave

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27 Frequency Distributions & Histograms When working with discrete data, the steps to construct a histogram are 1. Draw a horizontal scale, and mark the possible values. 2. Draw a vertical scale and mark it with either frequencies or relative frequencies (usually start at 0). 3. Above each possible value, draw a rectangle whose height is the frequency (or relative frequency) centered at the data value with a width chosen appropriately. Typically if the data values are integers then the widths will be one.

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28 Frequency Distributions & Histograms The number of lawyers in the firm will have the following histogram.

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29 Frequency Distributions & Histograms 50 students were asked the question, “How many textbooks did you purchase last term?” The result is summarized below and the histogram is on the next slide.

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30 Frequency Distributions & Histograms “How many textbooks did you purchase last term?”

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31 Frequency Distributions & Histograms Another version with the scale produced differently.

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32 Frequency Distributions & Histograms When working with continuous data, the steps to construct a histogram are 1. Decide into how many groups or “classes” you want to break up the data. Typically somewhere between 5 and 20. A good rule of thumb is to think having an average of more than 5 per group. 2. Use your answer to help decide the “width” of each group. I.e., You need to determine the width of the intervals that you are determining. 3. Determine the “starting point” for the lowest group.

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33 Example of Frequency Distribution Consider the student weights in the student data set. The data values fall between 103 (lowest) and 239 (highest). The range of the dataset is =136. There are 79 data values, so to have an average of at least 5 per group, we need 14 or fewer groups. We need to choose a width that breaks the data into 14 or fewer groups. Any width 10 or large would be reasonable.

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34 Example of Frequency Distribution Choosing a width of 15 we have the following frequency distribution.

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35 Histogram for Continuous Data 1. Mark the boundaries of the class intervals on a horizontal axis 2. Use frequency or relative frequency on the vertical scale.

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36 Histogram for Continuous Data The following histogram is for the frequency table of the weight data.

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37 Histogram for Continuous Data The following histogram is the Minitab output of the relative frequency histogram. Notice that the relative frequency scale is in percent.

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38 Histogram for Continuous Data Another version of a frequency table and histogram for the weight data with a class width of 20.

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39 Histogram for Continuous Data The resulting histogram.

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40 Histogram for Continuous Data Another version of a frequency table and histogram for the weight data with a class width of 20.

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41 Histogram for Continuous Data The corresponding histogram.

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42 Histogram for Continuous Data A class width of 15 or 20 seems to work well because all of the pictures tell the same story. The bulk of the weights appear to be centered around 150 lbs with a few values substantially large. The distribution of the weights is unimodal and is positively skewed.

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43 Illustrated Distribution Shapes UnimodalBimodalMultimodal Skew negativelySymmetricSkew positively

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44 Histograms with uneven class widths Consider the following frequency histogram of ages based on A with class widths of 2. Notice it is a bit choppy. Because of the positively skewed data, sometimes frequency distributions are created with unequal class widths.

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45 Histograms with uneven class widths For many reasons, either for convenience or because that is the way data was obtained, the data may be broken up in groups of uneven width as in the following example referring to the student ages.

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46 Histograms with uneven class widths If a frequency histogram is draw with the heights of the bars being the frequencies, the result is distorted. Notice that it appears that there are a lot of people over 28 when there is only a few.

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47 Histograms with uneven class widths To correct the distortion, we create a density histogram. The vertical scale is called the density and the density of a class is calculated by This choice for the density makes the area of the rectangle equal to the relative frequency.

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48 Histograms with uneven class widths Continuing this example we have

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49 Histograms with uneven class widths The resulting histogram is now a reasonable representation of the data.

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